Study Guide: Viruses – Structure, Replication, and Impact (Microbiology Ch. 5)

Viruses in the Biological Spectrum

Introduction to Viruses

Viruses are unique infectious agents that exist at the edge of life, being much smaller than bacteria and requiring host cells for replication. They are found everywhere in nature and can infect all forms of life, including bacteria, archaea, plants, animals, fungi, and protists.

• Obligate intracellular parasites: Viruses must invade and hijack a host cell to reproduce; they cannot multiply independently.

• Acellular microorganisms: Viruses are not composed of cells and lack cellular structures.

• Activity: Viruses are considered active only when inside a host cell; outside, they are inert particles.

• Classification: Viruses are classified separately from the three domains of life (Bacteria, Archaea, Eukarya) and are grouped into orders, families, genera, and species.

Structure of Viruses

General Structure

Viruses are ultra-microscopic, typically ranging from 10 to 150 nanometers. They are composed of a nucleic acid core (either DNA or RNA) surrounded by a protein coat called a capsid. Some viruses also have an outer envelope derived from the host cell membrane.

• Naked viruses: Consist only of a nucleocapsid (capsid + nucleic acid).

• Enveloped viruses: Possess an additional lipid envelope outside the nucleocapsid, often with embedded viral proteins (spikes).

• Capsid shapes: Can be helical, icosahedral, or complex.

Feature	Naked Virus	Enveloped Virus
Outer Layer	Capsid only	Capsid + lipid envelope
Stability	More stable in environment	Less stable, sensitive to detergents
Release from Host	Cell lysis	Budding or exocytosis
Examples	Poliovirus, Adenovirus	Influenza virus, HIV

Viral Genomes

• Viral genomes can be either DNA or RNA, and may be single-stranded or double-stranded.

• RNA viruses are further classified as positive-sense (can be directly translated) or negative-sense (must be transcribed to positive-sense before translation).

Modes of Viral Multiplication

Viruses that Infect Bacteria: Bacteriophages

Bacteriophages are viruses that infect bacteria. Their replication cycles are well-studied and serve as models for understanding viral infection.

• Virulent (lytic) phages: Undergo the lytic cycle, resulting in host cell lysis and release of new phages.

• Lysogenic (temperate) phages: Integrate their genome into the host DNA as a prophage, replicating along with the host cell until induced to enter the lytic cycle.

Steps in the Lytic Cycle

1. Attachment (adsorption): Phage binds to specific receptors on the bacterial surface.

2. Penetration: Phage injects its nucleic acid into the host cell.

3. Biosynthesis: Host machinery synthesizes viral components.

4. Assembly (maturation): New phage particles are assembled.

5. Release: Host cell lyses, releasing new phages.

Lysogeny and Lysogenic Conversion

• Lysogeny: Phage DNA integrates into the host genome as a prophage.

• Lysogenic conversion: Prophage genes may confer new properties to the host (e.g., toxin production in Vibrio cholerae and Clostridium botulinum).

Viruses that Infect Animal Cells: Animal Viruses

Animal viruses have similar replication steps to bacteriophages but with some differences due to eukaryotic cell structure.

1. Attachment (adsorption): Virus binds to specific host cell receptors.

2. Penetration: Entry by endocytosis or fusion (enveloped viruses).

3. Uncoating: Viral capsid is removed, releasing the genome.

4. Biosynthesis: Viral genome is replicated and proteins are synthesized.

5. Assembly: New virions are assembled.

6. Release: Virions exit the cell by lysis (naked viruses) or budding (enveloped viruses).

• Host range: Determined by the presence of suitable receptors on the host cell surface.

• Enveloped viruses: Typically enter by fusion and exit by budding, acquiring their envelope from the host cell membrane.

• Naked viruses: Usually enter by endocytosis and exit by cell lysis.

Viral Genome Replication Strategies

RNA Viruses

• Positive-sense RNA viruses: Genome acts as mRNA and can be directly translated by host ribosomes.

• Negative-sense RNA viruses: Genome must be transcribed into complementary positive-sense mRNA by a viral RNA-dependent RNA polymerase before translation.

• Retroviruses: Use reverse transcriptase to synthesize DNA from their RNA genome, which then integrates into the host genome (e.g., HIV).

DNA Viruses

• Most DNA viruses replicate in the host cell nucleus using host or viral DNA polymerases.

• Transcription and translation follow the central dogma: DNA → RNA → Protein.

Assembly and Release of Virions

• Virion assembly involves the spontaneous combination of capsid proteins and viral genomes.

• Release occurs by cell lysis (naked viruses) or budding (enveloped viruses).

• Enveloped viruses acquire their envelope from the host cell membrane during budding.

Pathogenesis and Effects of Viral Infection

• Cytopathic effects (CPE): Structural changes in host cells due to viral infection, such as cell lysis, syncytia formation, or inclusion bodies.

• Persistent infections: Virus remains in the host for long periods, sometimes for life (e.g., Herpesviruses).

• Latent infections: Virus remains dormant and can reactivate (e.g., Varicella-zoster virus causing shingles).

• Oncogenic viruses: Some viruses can cause cancer by transforming host cells (e.g., HPV, EBV).

Techniques in Cultivating Viruses

• Viruses require living cells for cultivation.

• Main methods: Inoculation into animals, embryonated eggs, or cell cultures.

• Plaque assay: Used to quantify viruses by counting clear zones (plaques) formed by virus-induced cell lysis in a cell monolayer.

Viruses and Human Health

• Viruses are important causes of human disease, including acute, chronic, and latent infections.

• Some chronic diseases of unknown cause may be linked to viral infections.

• Antiviral drugs target specific steps in viral replication, such as viral enzymes or entry mechanisms.

Key Terms

• Capsid: Protein shell surrounding the viral genome.

• Nucleocapsid: Capsid plus nucleic acid.

• Envelope: Lipid membrane surrounding some viruses.

• Prophage: Phage genome integrated into bacterial DNA.

• Reverse transcriptase: Enzyme that synthesizes DNA from RNA (retroviruses).

Additional info: The notes above expand on the original content by providing definitions, examples, and context for key microbiology concepts related to viruses, as well as clarifying the steps of viral replication and the differences between virus types.